As high-performance rare-earth sintered magnet, two kinds of magnets, i.e., an R—Co-based sintered magnet (R is mainly Sm) and an R-T-B-based sintered magnet (R is at least one kind of rare-earth element and absolutely includes Nd, and T is Fe or Fe+Co) are widely used.
Especially, since the R-T-B-based sintered magnet shows the highest magnetic energy product among various magnets and a price thereof is relatively low, this magnet is employed for various electric devices.
The R-T-B-based sintered magnet is mainly composed of: main phase comprising tetragonal compound of mainly R2T14B; R-rich phase; and B-rich phase. In the case of the R-T-B-based sintered magnet, basically, if an existence ratio of the tetragonal compound of R2T14B which is the main phase is increased, magnetic properties are enhanced. However, R easily reacts with oxygen in an atmosphere, and creates oxide such as R2O3. Therefore, if raw-material alloy for R-T-B-based sintered magnet or its powder is oxidized during a producing operation, the existence ratio of R2T14B is lowered, the R-rich phase is reduced and the magnetic properties are abruptly lowered. That is, if oxidization is prevented during the producing operation and an oxygen-containing amount of the raw-material alloy for R-T-B-based sintered magnet or its powder is reduced, the magnetic properties are enhanced.
The R-T-B-based sintered magnet is produced in the following manner. That is, a raw-material alloy is coarsely pulverized and finely pulverized to form alloy powder, the alloy powder is formed by pressing, and it is subjected to a sintering step and a thermal processing step. When the R-T-B-based sintered magnet is produced, in the process for coarsely pulverizing the raw-material alloy, since the pulverizing efficiency is high, hydrogen pulverizing operation is frequently used.
The hydrogen pulverizing operation is a technique in which hydrogen is stored in a raw-material alloy to make it brittle, thereby pulverizing the raw-material alloy, and this operation is carried out by doing the following steps.
First, an alloy which is raw material is inserted into a hydrogen furnace and then, an interior of the hydrogen furnace is decompressed by evacuation (vacuuming). Thereafter, hydrogen gas is supplied into the hydrogen furnace and the raw-material alloy is made to store hydrogen (hydrogen storing step). After predetermined time is elapsed, the raw-material alloy is heated (heating step) while evacuating the interior of the hydrogen furnace, and hydrogen is discharged from the raw-material alloy. Thereafter, the raw-material alloy is cooled (cooling step) and the hydrogen pulverizing operation is completed. According to this, the raw-material alloy is made brittle and coarsely pulverized powder is obtained.
The coarsely pulverized powder after the hydrogen pulverizing operation is pulverized into fine pulverized powder of a few μm.
Since the fine pulverized powder has a surface area greater than that of the coarsely pulverized powder, the fine pulverized powder is prone to be oxidized. Hence, oxidization of mainly the fine pulverized powder was conventionally prevented.
For example, there are proposed a technique (patent document 1) in which fine pulverized powder after fine pulverizing operation is put directly into mineral oil and then, the pulverized powder is formed, thereby lowering oxygen in sintered compact, and a technique (patent document 2) in which liquid lubricant is added to fine pulverized powder after fine pulverizing operation, surfaces of the particles are covered, thereby preventing oxidization of the fine pulverized powder. These methods propose to lower oxygen of fine pulverized powder.
As a method of obtaining a raw-material alloy for the R-T-B-based sintered magnet, strip casting is frequently used in recent years. In the strip castings, generally, raw-material alloy for R-T-B-based sintered magnet of 1 mm or less can be produced. Raw material alloy produced by the strip casting method is cooled relatively for a short time as compared with raw-material alloy produced by the conventional ingot casting method (mold casting method). Therefore, texture of the former raw-material alloy is refined, and crystal grain size is small. Thus, it is possible to obtain a sintered magnet having high magnetic properties as compared with the conventional sintered magnet. The raw-material alloy produced by the strip casting method has a large total area of grain boundary and has excellent dispersibility of R-rich phase. Hence, the raw-material alloy is prone to store hydrogen at the time of hydrogen pulverization and is prone to become brittle. Therefore, a particle diameter of the coarsely pulverized powder after hydrogen pulverization is small as compared with raw-material alloy produced by the conventional ingot casting. Further, since R-rich phases are dispersed, R-rich phase is prone to appear on the surface of particle, and the raw-material alloy is prone to be oxidized.
Heretofore, it is known that also in the case of coarsely pulverized powder having a relatively large particle diameter, if the coarsely pulverized powder comes into contact with an atmosphere during the producing operation, oxidization proceeds and an oxygen-containing amount is increased. However, since an increasing amount of oxygen is small as compared with fine pulverized powder, oxidization preventing measures are not taken almost at all. However, as the strip castings becomes popular as described above, it becomes necessary to prevent the oxidization also during coarsely pulverizing operation.
To prevent oxidization of coarsely pulverized powder (hydrogen pulverized powder) after hydrogen pulverizing operation, there is proposed a technique (patent document 3) in which a step in a recovery chamber for discharging hydrogen pulverized powder from a hydrogen pulverizer is carried out in inert gas.